Hot air impingement and microwave radiation are two different means for heating and cooking a food product based on different physical principles. Hot air impingement is based on the transfer of heat from a hot air having a higher temperature to an object having a lower temperature, changing the internal energy of the air and the object in accordance with the first law of thermodynamics. On the other hand, microwave radiation consists of electromagnetic waves having a typical wavelength of 12.24 cm or 4.82 inches and a frequency of 2,450 MHz, which are capable of causing dielectric heating of water, fat and sugar molecules in a food product.
Initially, ovens based on hot air impingement and microwave ovens were separately developed and commercialized. However, it was later demonstrated that a combination of hot air impingement and microwave radiation used in an oven can facilitate high-speed, high-quality cooking. See, for example, U.S. Pat. No. 5,254,823 to McKee et al., U.S. Pat. No. 5,434,390 to McKee et al., U.S. Pat. No. 5,558,793 to McKee et al., and U.S. Pat. No. 6,060,701 to McKee et al. This led to the development and commercialization of quick-cooking hybrid ovens based on both hot air impingement and microwave radiation and has established a new standard in the high-speed cooking technology.
While the technology of combining hot air impingement and microwave heating to achieve high-speed cooking in an oven has by now been well established, the current technology does not address a host of new challenges created by such combination, including the problem of inefficient energy use and consequent suboptimal cooking efficiency in the existing high-speed cooking ovens. The fundamental principle of cooking ovens is conversion of an available power (e.g., electric power) into heat energy to be directed to and absorbed by a food product in the oven to raise its internal temperature. Accordingly, the optimal cooking efficiency of an oven requires that the amount of heat energy converted from a given power be maximized; the amount of the heat energy directed to a food product in the oven be maximized; and the amount of the heat energy absorbed and retained by the food product be maximized. However, the current technology of the high-speed cooking ovens using both hot air impingement and microwave radiation is not directed to achieving such optimal cooking efficiency.
As a food product resides in a hot air environment of an oven, temperature gradients, or several boundary layers, form around the cooler food product. The oven cooks the food product by transferring the heat energy to the food product through these temperature gradients. Forced air convection by, for example, a fan can improve the heat transfer by “wiping away” the temperature gradients around the food product and bringing the higher temperature air closer to the food product. Hot air impingement can further improve the heat transfer by “piercing” the temperature gradients with jets of hot air and bringing the air at higher temperature closer to the surface of the food product. However, significant portions of the electric power and the heat energy from the hot air impingement are lost in the process to the oven walls, various openings, plenum and air blower that form the hot air circulation and delivery system of the oven. In addition, the presence of a microwave launcher in the cooking chamber may further reduce the efficiency of heat transfer by the hot air impingement.
Another well-known problem with the technique of hot air impingement is “spotting” in the areas directly impacted by the hot air jets, causing uneven heating or scorching of the surface of the food product. While this problem may be resolved by, for example, reduction in the hot air velocity and/or increase in the diameter of the columns of impinging hot air, such solutions may further reduce the efficiency of the hot air impingement.
In addition, the diameter/cross-sectional area of a column of hot air impingement generally increases as the distance from the hot air jet orifice increases, thereby reducing the efficiency of hot air impingement. While this problem may be solved by increasing the hot air velocity, as discussed above, such solution may further aggravate the spotting problem.
As for the microwave portion of the conventional high-speed cooking oven, a portion of the electric power is lost to heat within the transformer and magnetron during the process of generating microwaves. In addition, some portion of microwave energy is lost when reflected from the cavity walls back to the magnetron and dissipated through the cooling fan. This can occur when there is an uneven matching between the microwave delivery system and the microwave load.
Furthermore, it is also well known that microwave provides uneven heat energy distribution across the volume of a cooking cavity. While the horizontal unevenness may be eliminated by rotating the food product around a vertical axis in the oven, as many conventional microwave ovens do, such solution does little to reduce the vertical unevenness in the heat energy distribution.
There is yet another source of inefficiency in the conventional high-speed cooking oven. Until the temperature at any portion of a food product in the oven reaches 212° F. at which the water molecules in the food product start being converted into steam during the cooking process, the amount of the energy absorbed by the food product roughly equals the amount of the energy directed at the food product. However, after the point when the water starts to be converted into steam, a portion of the energy directed at the food product is not absorbed by the food product, but is lost as the energy of activating the water into steam, which subsequently escapes from the food product carrying away a portion of thermal energy from the food product. This phenomenon is further complicated by the fact that the heat energy absorbed at the surface of the food product is not immediately dispersed downward below the surface due to the finite heat transfer coefficient (or thermal conductivity) of the food product and it takes some time to bring the inner mass of the food product into thermal equilibrium with the surface. Accordingly, the efficiency in heat transfer to the food product in the oven decreases after the temperature of the food surface reaches 212° F., when the resulting steam at a higher temperature than the inner temperature of the food product carries away heat energy from the food product.
Yet another potential source of inefficiency exists in conventional high-speed cooking ovens. In prior art ovens, at least one side wall, and typically both side walls, of the cooking chamber is exposed to either the outside surface of the oven (with perhaps insulation or microwave waveguides disposed between the wall of the cooking chamber and the outside surface of the oven) or to the controls of the oven.
In summary, the problem with the current high-speed cooking technology based on a combination of hot air impingement and microwave radiation is that the combination has never been done in a way to optimize the cooking efficiency of the oven. With the suboptimal cooking efficiency in the presence of various sources of inefficiencies in the conversion of electrical power to heat, the currently available high-speed cooking ovens (either commercial models or residential models) operate on a power supply based on 220 volts or greater. As a result, this relatively high electric power required to operate the high-speed cooking oven limits the universe of possible applications and customer bases, especially in residential households where a 120 volt-based power supply is more common.
Thus, it is an object of the present invention to eliminate or reduce some of the inefficiencies in heat transfer present in the conventional high-speed cooking ovens.
It is yet another object of the present invention to optimize the cooking efficiency of a high-speed cooking oven.
It is yet another object of the present invention to optimize the combination of hot air impingement and microwave to seek the greater cooking efficiency than was possible in the conventional high-speed cooking oven.
It is yet another object of the present invention to optimize the cooking efficiency of the hot air impingement.
It is yet another object of the present invention to optimize the cooking efficiency of the microwave heating.
It is yet another object of the present invention to resolve the spotting problem without compromising the cooking efficiency of the hot air impingement.
It is yet another object of the present invention to provide a more even distribution of microwave heating compared to the conventional high-speed cooking oven.
It is yet another object of the present invention to match the cavity of a high-speed cooking oven to the microwave load.
It is yet another object of the present invention to optimize the efficiency of heat transfer to a food product in the oven by overcoming the inefficiency created by the heat loss due to the water steam escaping from the food product at 212° F. and the time lag in the heat energy distribution in the inner mass of the food product due to its finite heat transfer coefficient.
It is yet another object of the present invention to provide a high-speed cooking oven that can operate on a power supply based on voltage less than 220 volts.
It is yet another object of the present invention to provide a high-speed cooking oven that can operate on a power supply based on a voltage between 110 and 125 volts.
It is yet another object of the present invention to provide a high-speed cooking oven capable of operating on a power supply based on the voltage of 120 volts and the current of 30 amperes.
It is yet another object of the present invention to reduce the operating costs of high-speed cooking ovens.
Other objects and advantages of the present invention will become apparent from the following description.